1. Field of the Invention
The invention relates to a method of providing the facility for some users of a communication network to have preferential access for particular calls.
2. Description of the Prior Art
For various reasons, some users of a communication network are granted priority access for one or more particular calls, so that these preferred subscribers can obtain said calls under the best possible conditions, in particular if the traffic handled by the network corresponds to a load such that obtaining such calls in the normal way could not otherwise be guaranteed.
A prior art solution to ensuring high availability of a communication network for this kind of user is to allocate them circuits permanently, a pair of preferred users having a circuit reserved for them at all times on trunks between network switching nodes which are “borrowed” to interconnect the users concerned. Each circuit is constituted on a path or one of the available paths between the switching nodes to which the users concerned are connected. That path consists either of a reserved segment chosen from the segments that coexist on a trunk or of a plurality of reserved segments, each of which is chosen from one of the trunks via which the chosen path is set up, and which are connected in series by switching nodes of the network at which the trunks including the chosen segments terminate. Permanently reserving a circuit is a costly solution and is generally used only for a limited number of pairs of preferred users of a given network. It is less than the optimum, in particular when the circuits reserved for the preferred users are not being used by them and the traffic relating to other users is strongly retarded by the level of occupation of a trunk including the reserved segments.
Another prior art solution avoids the drawback of permanently reserving circuits for preferred users. It consists in the facility to cut off a call set up between users who are not preferred users if the conditions are such that there is no other way to set up a circuit enabling preferred users to communicate with each other. This solution represents a significantly lower penalty if the capacity of the network is sufficiently high to carry the traffic under good conditions and recourse to the pre-emptive rights of preferred users does not normally have to be used, and can therefore be considered as a last resort in the event of a problem. This solution is not satisfactory, however, if the pre-emptive right needs to be exercised, as this entails interrupting a call between users who have not requested this.